Women who start periods early likelier to have girls

WILL a baby be a boy or a girl? If the mother started her period at a young age, it is more likely to be a girl.

That's according to Misao Fukuda at the M&K Health Institute in Hyogo, Japan, and colleagues, who found subtle differences in sex ratios of children depending on when a mother entered menarche.

Fukuda asked over 10,000 mothers the age at which they had begun their period and the sex of their baby. Forty six per cent of the children born to women who began their periods at age 10 were boys. This figure rose to 50 per cent when the woman began her period at 12, and 53 per cent when the women entered menarche at age 14 (Human Reproduction,DOI: 10.1093/humrep/der107).

Fukuda points to previous research demonstrating higher levels of the female sex hormone oestradiol in women who entered menarche before the age of 12. This may lead to spontaneous miscarriage of fertilised male eggs, he says. The theory is plausible, says Valerie Grant at the University of Auckland, New Zealand, as male embryos are known to be more vulnerable to hormone imbalances.

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